Leviathan is a large beast of the sea that is referenced in the Old Testament, The word has become synonymous with any large sea creature.
Leviathan has the same origin as the Old Testament gives, but he is more or less a demon of pride. Leviathan is one of the most powerful demons there is, and high in the hierarchy.
Mood toward humans: Leviathan is not to welcoming of humans, and for good reason. We do not know much about Leviathan or how he operates, but we do know he is immortal, was never killed and invincible in every way we have heard about. The hellmouth is not to be taken lightly.
The Leviathan of the Middle Ages was used as an image of Satan endangering both God's creatures—by attempting to eat them—and God's creation—by threatening it with upheaval in the waters of Chaos. St. Thomas described Leviathan as the demon of Envy, first in punishing the corresponding sinners. Leviathan became associated with, and may originally have referred to, the visual motif of the Hellmouth, a monstrous animal into whose mouth the damned disappear at the Last Judgement, found in Anglo-Saxson art from about 800, and later all over Europe.